In order to reset to zero a counter such as a chronograph counter, a heart-shaped reset cam is associated with the arbour of the counter. This cam is associated with a hammer that has an impact surface which, when it abuts the symmetrical shoulders of the cam, enables the hammer to immobilise the heart cam in an angular position corresponding to zero on the counter. If, because of the working of the chronograph mechanism, the heart shaped cam is moved away from the angular position thereof corresponding to zero on the counter and the hammer strikes the cam, said cam will rotate via the effect of the force of the hammer and return to the angular position thereof corresponding to zero on the counter. In this position, the impact surface of the hammer abuts the two shoulders of the heart shaped cam and immobilises the cam.
A lever with two hammers is used for simultaneously resetting to zero two counters. Given the different tolerances, this device does not, in practice, enable the two impact surfaces of the hammers to abut simultaneously the respective shoulders of the two heart cams in the zero reset position of the two counters, such that the zero position of one of the counters is not defined precisely.
In order to take account of these tolerance problems, the lever with two hammers has been allowed a slight degree of pivoting freedom when it reaches the end of travel. It will be understood that allowing the lever to pivot slightly when the impact surfaces of its hammers are abutting against the respective shoulders of the two heart cams enables the hammer to adapt to a slight shift between said heart cams and to return said cams to the angular position thereof corresponding to zero on the counters. Nonetheless, when the lever pivots, this generates friction forces which are added to the resistant forces raised by the various elements of the chronograph mechanism that are set in action when the user presses on the control push button. Thus, the result of forces may be such that the action exerted by the user on the control push button becomes less than the reaction of the various mechanical elements that are set in operation via the effect of said action. The resulting situation is such that the action of the user has no effect and the counters are immobilised in an uncontrollable position. Consequently, anything that can help to decrease the reaction forces opposed by the chronograph mechanism when the latter is operated, must be examined. Thus, it is more advantageous to try to reduce the friction forces that oppose the movement of the lever with two hammers.